Monoclonal antibodies have been developed and cDNAs isolated to a nucleolar antigen (p120) that is detected in rapidly proliferating cells but not in normal resting cells nor in many benign and slow growing malignant tumors. In tumor cells, p120 is found in a novel DNA associated fibril-like particle that is localized to a unique perinucleolar region. In cancer cells the nucleolus in "hyperactive" and has a 15-100 fold greater rRNA synthesis than in normal resting cells. The objectives of this study are to determine whether p120 plays a role in this increased nucleolar activity. The specific aims of this study are: to purify and characterize p120 and its related nucleolar particle; to study the function of p120; and to assess the regulation of p120 transcription and correlate p120 transcription levels to cell cycle potential. P120 will be purified from its DNA associated nucleolar particle and amino acid composition, partial amino acid sequence and DNA binding properties will be determined. Inasmuch as p120 is proliferation related we will further characterize proteins of the p120 particle and determine whether these peptides are preferentially expressed in growing ells. The function of p120 will be studied by assessing cell cycle progression, cytomorphological changes, and rates of DNA and rRNA synthesis, following the microinjection of p120 antibodies or transfection of p120 antisense transcripts. Function will also be evaluated following the addition of p120 antibodies or purified protein to in vitro assays that measure nucleolar activation, rDNA synthesis and rRNA transcription. P120 transcription levels and transcription rates will be correlated with cell cycle progression and the proliferative state of the cell. Studies will be done to determine whether the regulation of p120 transcription differs between tumor cells and normal proliferating cells. These studies will provide insight into the function of p120; particularly the possible role of p120 in the maintenance of the rapid proliferative state found in some cancer cells and in further assessing the structural-functional relationship of the p120 related nucleolar particle and its component parts with nucleolar activity and with the proliferation state of the cell.